Plasma arc cutting process and apparatus using an oxygen-rich gas shield

ABSTRACT

A plasma arc torch has a secondary gas flow that is extremely large during piercing of a workpiece to keep splattered molten metal away from the torch and thereby prevent &#34;double arcing&#34;. The secondary flow exits the torch immediately adjacent the transferred plasma arc and is an extremely uniform, swirling flow. A swirl ring is located in the secondary gas flow path at the exit point. A prechamber feeds gas to the swirl ring, which is in turn fed through a flow restricting orifice. For certain applications the secondary gas is a mixture of an oxidizing gas, preferably oxygen, and a non-oxidizing gas, preferably nitrogen, in a flow ratio of oxygen to nitrogen in the range of 2:3 to 9:1. Preferably the flow ratio is about 2:1. A network of conduits and solenoid valves operated under the control of a central microprocessor regulates the flows of plasma gas and secondary gas and mixes the secondary gas. The network includes valved parallel branches that provide a quick charge capability and a set of venting valves, also electrically actuated by the microprocessor, to provide a quick discharge. In a preferred high-definition embodiment, a nozzle with a cut back outer surface and a large, conical head allows a metal seal and enhanced cooling. A two-piece cap protects the nozzle during cutting.

REFERENCE TO RELATED APPLICATIONS

This application is a division of Ser. No. 07/753,395 filed Aug. 30,1991, now U.S. Pat. No. 5,396,093, which is a continuation-in-part ofU.S. Ser. No. 07/395,266 filed Aug. 17, 1989, now U.S. Pat. No.5,120,930, which in turn is a continuation-in-part of U.S. Ser. No.07/203,440 filed Jun. 7, 1988 now U.S. Pat. No. 4,861,962 issued Aug.29, 1989. This application is also a continuation-in-part of U.S. Ser.No. 07/682,991 filed Apr. 12, 1991, now U.S. Pat. No. 5,170,033, andU.S. Ser. No. 07/682,992 also filed Apr. 12, 1991, now U.S. Pat. No.5,166,494, both of which are in turn continuations-in-part of U.S. Ser.No. 07/513,780 filed Apr. 24, 1990, now U.S. Pat. No. 5,070,227.

BACKGROUND OF THE INVENTION

This invention relates in general to plasma arc cutting and weldingprocesses and apparatus. More specifically, it relates to a process andapparatus for dual flow piercing and cutting of metal workpieces that isfaster, has a better cut quality, and protects the torch againstsplattered molten metal through the use of a high velocity gas secondarygas flow of well-defined flow conditions and a novel composition.

Plasma arc torches have a wide variety of applications such as thecutting of thick plates of steel and the cutting of comparatively thinsheets of galvanized metal commonly used in heating, ventilating and airconditioning (HVAC) systems. The basic components of a plasma arc torchinclude a torch body, an electrode (cathode) mounted within the body, anozzle (anode) with a central exit orifice, a flow of an ionizable gas,electrical connections, passages for cooling and arc control fluids, anda power supply that produces a pilot arc in the gas, typically betweenthe electrode and the nozzle, and then a plasma arc, a conductive flowof the ionized gas from the electrode to a workpiece. The gas can benon-oxidizing, e.g. nitrogen, argon/hydrogen, or argon, or oxidizing,e.g. oxygen or air.

Various plasma arc torches of this general type are described in U.S.Pat. Nos. 3,641,304 to Couch and Dean, 3,833,787 to Couch, 4,203,022 toCouch and Bailey, 4,421,970 to Couch, 4,791,268 to Sanders and Couch,and 4,816,637 to Sanders and Couch, all commonly assigned with thepresent application. Plasma arc torches and related products are sold ina variety of models by Hypertherm, Inc. of Hanover, N.H. The MAX 100brand torch of Hypertherm is typical of the medium power torches (100ampere output) using air as the working gas and useful for both platefabrication and HVAC applications. The HT 400 brand torch is typical ofthe high power torches (260 amperes) often using oxygen as the workinggas. High power torches are typically water cooled and used to pierceand cut thick metal sheets, e.g. 1 inch thick mild steel plate.

Design considerations of these torches include cooling the torch sincethe arc produces temperatures in excess of 10,000° C. which if notcontrolled cold destroy the torch, particularly the nozzle. Anotherconsideration is that the arc must be controlled, both to protect thetorch itself from the arc and to enhance the quality of the cut beingmade in a workpiece. An early invention of one of the present applicantsdescribed in U.S. Pat. No. 3,641,308 involved the use of a flow ofcooling water in the nozzle of a torch to constrict the arc and therebyproduce a better quality cut. It has also been found that the cutquality can be greatly enhanced if the plasma is caused to swirl, as byfeeding it to the plasma chambers through a swirl ring having a set ofoff-center holes.

In cutting parts from sheet metal, a cut often begins by piercing thesheet at an interior point. Because the metal is not cut through whenthe piercing begins, the molten metal cannot run out of the kerf underthe force of gravity. It is therefore splashed upwardly onto the torch.This is undesirable because the metal can destabilize the arc, causingit to gouge the nozzle, and it can adhere to the nozzle, which willoften lead to double arcing, where the plasma arc flow from theelectrode to the nozzle, and then to the workpiece via a conduction pathof molten metal. Both gouging and double arcing reduce the nozzle life,or destroy it. It is also important that the resulting cut be smooth, asfree of dross as possible, and have a cut angle that is preferably at ornear 0°, that is, with the "good" side of the kerf having a surface thatis perpendicular to the metal sheet itself.

In the past, to control gouging and double arcing due to splatteredmetal, the solution for high current (200 amperes or more) torches hasbeen to use a multi-piece nozzle with water injection cooling. Typicalsuch nozzles sold by Hypertherm, Inc. are illustrated in schematic formin FIGS. 1A and 1B. Hypertherm Model Nos. HT400 0.099, HT400 0.166 andPAC500 0.187 correspond to FIG. 1a and use a ceramic nozzle face cooledby water. FIG. 2B shows a variation on this design which is sold byHypertherm, Inc. as its Model PAC500 0.250.

For low current operation, 0-200 amperes, water injection cooling isless practical due to its cost and the energy drain from the plasma bythe water cooling. The common commercial solution for low power, aircooled torches was simply to allow the metal to attach the torch partand then replace them. A typical nozzle life such for such a torchoperating at 40-50 amperes when piercing and cutting 1/4 inch mild steelis about one hour. There is clearly a cost associated with thereplacement parts, the productive time lost during the replacementprocess, as well as safety considerations that arise whenever a torch isdisassembled and reassembled.

Gas cooling of nozzles is also known. It usually involves a dual flow,that is a primary flow of a plasma gas and a secondary flow. They canoriginate at a common inlet, or separate inlets. The primary flow mustbe formed by an ionizable gas; the secondary flow is not necessarilyionizable. The primary flow passes through the plasma chamber where itis ionized and exits the torch through its nozzle to form a plasma jet.The secondary gas flows outside the nozzle to form a cold layer ofnon-ionized gas around the arc. In conventional torches the temperatureand velocity of the primary or plasma gas are much higher than those ofthe secondary gas flow.

While the cutting capabilities of the torch are principally a functionof the plasma jet, the secondary flow can be important to cool the torchand to create a protected gaseous environment at the workpiece. FIG. 2Ashows a typical use of a secondary flow of gas over the outer surface ofa nozzle toward the workpiece. This arrangement is used for low currentapplications; nozzles of this type are sold by Hypertherm, Inc. as itsmodel Nos. HT40 0.038 and MAX100 0.059. FIG. 2B show another gas coolingarrangement with a ceramic insulating sleeve at the lower end of thenozzle to protect the nozzle against contract against the workpiece. Theceramic, however, is brittle and this arrangement offers no protectionof the nozzle during piercing.

U.S. Pat. No. 4,389,559 to Rotolico et al. and U.S. Pat. No. 4,029,930to Sagara et al. are examples of plasma torches for underwater sprayingand welding applications, respectively where a sheath of secondary gasshields the zone where the arc is acting against the surroundingatmosphere, whether air or water. U.S. Pat. No. 4,816,637 to Sanders andCouch discloses a high current underwater cutting torch with an inwardlydirected radial flow of air at 0 to 10 scfm in combination with anannular water sheath to create a water-free cutting zone and to sweepaway hydrogen gas that would otherwise accumulate under the workpiece.

As noted above, the ability of a plasma torch to pierce is veryimportant in a plasma cutting process. The commonly assigned U.S. Pat.No. 4,861,962 to Sanders and Couch discloses the use of a metallic,electrically floating shield that substantially surrounds the nozzle toblock metal splattered on piercing. A secondary gas flow between theshield and the nozzle cools these components. Canted ports upstreamintroduces a swirl into the secondary flow to help stabilize the arc andimprove the cut quality. Bleed ports in the shield also draw off aportion of the cooling flow to allow an increased overall flow forbetter cooling without destabilizing the arc during cutting. Thissolution is, however not adequate for high-definition (sometimes termedhigh-density) torches which have a concentrated arc and require morecooling than a gas can provide. The secondary flow is relatively low inorder to maintain the cut quality. The gas functions to cool the torchand to assist in stabilizing the arc.

In dual flow torches, when the primary gas is oxygen or air, thesecondary gas is usually air. When the primary gas is nitrogen, thesecondary gas is usually carbon dioxide or nitrogen. These combinationsproduce a suitable plasma jet without an unacceptable level ofinterference by the secondary gas with the cut. With these secondarygases, the kerf usually exhibits a positive cut angle of 1 to 2 degreesand top and bottom dross. Cut speed and quality are otherwise about thesame as if no shield was used.

It is also known to provide different gases, or mixes of gases, fordifferent phases of the cutting operation. For example, JapanesePublished Document No. 57-68270 of Hitachi Seisakusho K.K. discloses apreflow of argon during a pilot arc phase, and a switch to hydrogen gasfor the cutting, followed by a return to argon after the cutting isterminated. Japanese Published Application No. 61-92782 of Koike OxygenIndustry, Inc. which discloses a nitrogen-oxygen mix as a preflow plasmagas on start up, followed by an oxygen plasma flow. Both of these flowsare for the plasma gas, not a secondary gas. This publication teachesthat a plasma or primary gas preflow of about 85% nitrogen, 15% oxygenis best to extend electrode life. U.S. Pat. No. 5,017,752 to Severanceet al. discloses a flow of a non-oxidizing gas during pilot arcoperation which is switched to a pure oxygen flow when the arctransfers. These flows are, again, of primary gas only. Various patentsand publications also disclose patterns of gas flow and timingconsiderations. U.S. Pat. No. 4,195,216 to Beauchamp et al., forexample, discloses various modes of operating a plasma-wire welder in amanner that fills the keyhole at the end of the weld by adjusting thewire feed rate in coordination with changes in the gas flow and the arccurrent.

Applicants are not aware of a torch where an extremely high velocityflow of a secondary gas is used as a gas shield to protect the nozzleand other torch components adjacent the workpiece against splatteredmolten metal on piercing. Heretofore the lack of uniformity of the flowand flow hysteresis have made the direct interaction of a high velocitygas flow with the plasma jet a situation to be avoided. Applicants arealso not aware of the use of a mixture of gases as a secondary gas flowin order to speed the cut and/or increase the cut quality adjustablythrough a change in the mix of gases forming the secondary gas. Inparticular applicants are not aware of any secondary gas flow using amixture of nitrogen and oxygen where the ratio of gases in the mixtureis opposite to that of air. Applicants are also not aware of a highdefinition plasma arc torch that uses a gas shield, this mixture ofsecondary gases, or flow controls that allow sudden, precise and largechanges in the gas flow rates through the torch.

It is therefore a principal object of this invention to provide a plasmaarc torch and method of operation that protects the torch againstgouging and double arcing during piercing.

Another principal object of this invention is to provide a plasma arctorch and method of operation which increases cutting speed and producesa kerf of enhanced cut quality.

A further object of this invention is to provide the foregoingadvantages for a high-definition torch.

Another object is to provide the foregoing advantages, including a cutthat has a smooth side surface, a good cut angle, and is substantiallyfree of top dross.

Still another object is to provide the foregoing advantages and also theability to adjust the cutting operation to adapt to different materialsand cutting requirements depending on the application without anychanges in equipment.

SUMMARY OF THE INVENTION

A plasma arc cutting system according to this invention has a dual gasflow, with a secondary flow at an extremely high rate, e.g. 120 scfh,during a piercing of a sheet metal workpiece, as compared to a typicaloperating flow rate of 20 scfh. The high velocity secondary flow isdirected radially inwardly onto the arc. This flow is characterized byan extreme uniformity in time and space, a swirling flow pattern, andthe close positioning of an annular exit orifice with respect to thetransferred arc. The secondary flow is preferably a mixture of oxygenand nitrogen. At least 40% of the flow is oxygen, and the flow rates arein a range of flow rate ratios of about 2:3 to about 9:1. Preferably theflow rate ratio is about 2:1. The plasma gas flow for a high definitiontorch with a rating of 15 amperes is typically 7 scfh. The presentinvention also includes primary and secondary gas flow controls thatallow a quick charging and discharging of the flow lines in order toaccommodate sudden large changes in flow rates without loss of controlover the arc.

The plasma arc torch has a secondary gas cap mounted on its lower endwith a front face interposed between a nozzle mounted on the torch andthe workpiece. In the preferred form of high definition torch, awater-cooled cap is mounted between the nozzle and the secondary gas capto define a water cooled chamber adjacent the outer surface of thenozzle for high efficiency cooling. A swirl ring is mounted between thewater cooled cap and the secondary gas cap immediately upstream of theannular exit orifice. It contains a set of canted holes that introduce aswirl in the gas passing through it. A prechamber is upstream of theswirl ring, fed by a flow restricting orifice to create a pressure dropin the secondary gas feed line across the water-cooled cap. Thispressure drop, prechamber and downstream swirl ring produce the flowcharacteristics of the present invention.

The nozzle is characterized by a large head that surrounds an exit portfor the plasma jet and a sharp cut back or recess to a conical bodyportion. This nozzle design promotes cooling of the nozzle and allows areliable metal-to-metal seal of the nozzle to a water-cooled cap, orequivalent component. The secondary gas cap has a first, generallycylindrical portion that mounts on an insulating member, a transitionportion that inclines toward the plasma jet, and a replaceable faceportion that extends over the lower end of the torch, opposite theworkpiece, with a central port aligned with the exit port of the nozzleand closely surrounding it. Preferably the face portion has a set ofbleed/vent ports angled away from the jet, a locating and mountingrecess at its outer edge, a groove to hold an o-ring seal, and alocating groove for the swirl ring.

The flow controls of the present invention include a microprocessorcontrolled network (or "circuit") of conduits, valves, meters, and ventsthat provide a primary gas and a mixed secondary gas in variable ratiosof two gases at multiple preselected flow rates, e.g. a preflow and anoperating flow. In a preferred form oxygen and nitrogen supply lineseach feed a flow meter that makes the flow rate independent of theupstream pressure. The oxygen supply flows to the plasma gas line and toa secondary gas circuit. These two oxygen flow lines and one nitrogenflow line in the secondary circuit each has a solenoid actuated flowmeter bypass valve, followed by three parallel branches that each haveanother solenoid actuated valve and a needle valve. One branchestablishes a preflow. A second branch establishes an operating flow.The third branch allows a sudden increased flow of gas to provide a"quick charge". This quick charge is due to a flow path that bypassesthe flow restricting valves in the other branches.

The output of the oxygen and nitrogen secondary gas lines are combinedinto a single secondary feed conduit leading to the secondary gas inletat the torch. This feed conduit and the primary and secondary gas feedlines adjacent the torch are vented to atmosphere through a solenoidactuated three way valve. Opening of the two vents in the secondarygas-line briefly during the transfer from a pilot arc mode to atransferred arc mode allows the secondary gas flow to drop quickly toits operating value for cutting. Opening all three vents on plasma cutoff provides a quick discharge of the gas flows to the torch. In orderto have a strong secondary gas flow throughout piercing, there is a timedelay between the transfer of this plasma to the workpiece and theswitching from the preflow to the operating flow of the secondary gas.

These and other features and objects of the present invention will bemore fully understood from the following detailed description whichshould be read in light of the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a simplified view in vertical cross section of a prior artelectrode and multi-piece nozzle of a high-current, water-injectionplasma arc torch;

FIG. 1B is a view corresponding to FIG. 1A of an alternative prior artmulti-piece, water injection nozzle;

FIG. 2A is a simplified view in vertical cross section of a prior artone-piece nozzle of a plasma arc torch for use with low currents;

FIG. 2B is a view corresponding to FIG. 2A of an alternative prior artone-piece nozzle embodiment for low current use using a cylindricalceramic shield;

FIG. 3A is a view in vertical section of a high definition water and aircooled plasma arc torch according to the present invention which showsthe plasma gas and secondary gas passages;

FIG. 3B is a view in vertical section of the present invention showingthe water cooling passages;

FIG. 3C is a detailed view in vertical section of the nozzle and exitport area of the torch shown in FIG. 3A;

FIG. 3D is a view in horizontal section of the swirl ring shown in FIG.3A;

FIG. 4 is a schematic flow control circuit according to the presentinvention providing a mixed gas secondary gas flow at varying flow ratesand with a quick charge and quick discharge capability; and

FIG. 5 is a timing diagram for the control circuit shown in FIG. 4.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 3A and 3B show a plasma arc torch 10 according to the presentinvention. It has a multi-component body 12 including a generallycylindrical main body portion 12a formed of an insulating material suchas FR4 fiber glass or Delrin. Anode block 14 secured in the body portion12a has an opening 14a that receives a plasma gas conduit 16 and anopening 14b that receives secondary gas conduit 18, both the plasma gasconduit 16 and the secondary gas conduit 18 pass through an insulatorblock 20. A nozzle 28 is mounted immediately below an electrode 24 in aspaced relationship to define a plasma arc chamber 30 therebetween whereplasma gas fed from a swirl ring 32 is ionized to form either a pilotarc between the electrode and the nozzle or a transferred arc, or plasmajet, 34 between the electrode and a workpiece 36. The jet 34 pierces theworkpiece and then cuts a kerf 38. Note that swirl 32 is comprised oftwo pieces 32a and 32b. Radial ports 32c on swirl ring port 32adistribute the plasma gas flow evenly to injection ports 32d on swirlring port 32b. The electrode 24 has a hafnium insert 24a.

As shown, the nozzle has a configuration specially adapted for a highdefinition torch with a narrow exit port 28a, a large diameter nozzlehead 28b to act as a good heat sink, a severe cutback or recess 28c, anda conical body portion 28d. This design provides good heat transfer andtherefore cooling of the nozzle by water circulated over the outside ofthe nozzle. It also facilitates a reliable metal-to-metal seal at 66abetween the nozzle head and a like inclined end surface of awater-cooled cap 66. The various component parts are assembled withfluid tight seals provided by sets of o-rings each seated in anassociated annular groove, and the metal seal 66a.

A gas source 42 provides a flow of a plasma gas through a primary gascontrol circuit 44a (FIG. 4) to a plasma gas inlet 10a of the torch 10.A source 46 of a second gas flows through a flow control circuit 44b toa secondary gas inlet 10b of the torch. The secondary gas in thepreferred form shown includes a mix of gases from both sources, as isdiscussed in more detail below. In the torch the plasma gas follows aflow path 48 that includes tube passage 16a, vertical passage 48a,radial port 48b to the swirl ring 32 and then to the plasma chamber 30where it is ionized. The secondary gas follows a flow path 50 thatincludes tube passage 18a, a vertical passage 52, a radial port 54, aflow restricting orifice 56, a prechamber 58, a secondary gas swirl ring60 and an annular exit orifice 62.

This secondary flow path, and in particular orifice 56, prechamber 58and the swirl ring 60 is a principal feature of this invention. Itintroduces a high degree of flow uniformity and control over the flow ata point immediately adjacent to the transferred plasma arc 34. The swirlring 60 contains a set of off-center, or canted holes 64 which introducea swirling movement to the flow which facilitates the interaction of thesecondary gas stream with the jet 34 and has a beneficial effect on thecut quality. The swirl ring is formed of an insulating material such asa high temperature plastic, preferably the product sold by I.E. du Pontde Nemours under the trade designation Vespel. As shown, the exitorifice 62 has a flat annular portion 62a, a conical portion 62bdirected downwardly and radially inward, and a final flat annularportion 62c that is generally parallel to the workpiece 36. The orificepassages 62b and 62c mirror the outer dimensions of the adjacent nozzlesurfaces.

The prechamber 58 acts as a local gas supply to the swirl ring 60. Theflow restricting orifice 56 creates a pressure drop at the opposite endof the prechamber 58 from the swirl ring. The orifice 56 and prechamber58 isolate the swirl ring from upstream pressure and flow ratefluctuations. To draw an electrical analogy, the orifice 56 andprechamber 58 act as a smoothing capacitor in an a.c. circuit. On shutoff, when the arc current is cut off, the gas in the plasma chambercools rapidly, leading to a sudden outrush of gas. Gas in the secondaryflow path, absent this invention, would be drawn out in this outrush bythe Venturi effect. However, the orifice 56 choke off the outrush sothat only the comparatively small supply of gas in the prechamber 58 isdrawn out. This supply is calculated to continue the arc stabilizationof the secondary gas during cut off, but to have the secondary gas flowcease generally coincident with the extinction of the arc. Thisarrangement provides a secondary flow from the exit orifice 62 which ishigh uniform, both in time and spatially.

In the high definition torch of FIGS. 3A-3D, the arc is highlyconstricted as compared to conventional plasma arcs. It also has a highenergy density. In a standard plasma cutting torch the current densityis approximately 25,000 Amp/sq. inch; in a high density plasma thecurrent densities can be as high as 80,000 Amp/sq. inch, measured in thenozzle base. A 15 ampere current is typical. Water cooling has beenfound to be necessary. To this end, the water-cooled cap 66 is threadedinto the lower end of the anode block 14, with an o-ring seal at 68 andthe face-abutting metal-to-metal seal 66a to the upper edge of thenozzle head 28b. Water flow 45a is passed through a water chamber 70defined by the cap 66, the outer surface of the nozzle 28 and the lowerend of the anode block 14. The cooling water 45 flows into the torchthrough passages 47 which includes water inlet tube 17 which is fittedinto opening 15a in the cathode block 15. Water flows from the tubeoutlet 47a through annulus 47b, radial holes 47c in both the cathodeblock 15 and insulator 13, annulus 47d, radial holes 47e, annulus 47f tothe drill holes 47g. Here the flow splits into two flows 45a to thenozzle and 45b to the secondary cap via vertical passage 47h and annulus47i respectively. Flow 45a returns from chamber 70 via vertical passage47j which joins returning flow 45b at hole 47k then flows out of thetorch through tube conduit 19 which is fitted to nozzle block 14 atopening 14c.

Another principal feature of this invention is a secondary gas cap 72threaded at 74 to the insulating body and gas sealed by o-rings 40c and40d to the body. The secondary gas cap has a first portion including acylindrical body 72a terminating in conical wall portion 72b with a step72c in its side wall. A second or face portion 72d includes a step 72ethat mates with step 72c, a groove 72f that holds o-ring 40e, vent ports72g, a recess 72h that holds and positions the swirl ring 60 at itslower edge, an exit orifice 72i centered on the nozzle exit orifice andclosely spaced around the plasma jet, and wall portions 72j, 72k and 72lthat mirror the nozzle in a parallel spaced relationship and definetogether with the nozzle the exit orifice 72i.

The cap 72 is in a parallel spaced relationship with the cap 66 with thegap between them defining the prechamber 58. The secondary gas cap notonly defines the secondary flow path, it also acts during piercing as amechanical shield against splattered metal. The lower portion of thecap, particularly the face piece 72d, intercepts any molten metalsprayed upwardly that is not swept away by the gas shield of the presentinvention, that is, a strong shielding flow of secondary gas thisimpinges on the plasma jet and is turned to flow radially outwardlybetween the cap 72 and the workpiece. Note that the central exit orifice72i has a very small diameter to closely surround the plasma jet 34 withas small a clearance as is possible without risking gouging. The shieldis also electrically floating. It is mounted on an insulating material,the body part 12a, and is spaced from adjacent metallic members such asthe nozzle 28 and water cooled cap 66, and the swirl ring 60 is formedof an insulating material. As a result, should any molten metal adhereto it, it will not be part of a conductive path for a double arcing. Thevents 72g encircle the exit orifice 72i. They are sized and numbered sothat during the cutting operation of the torch, they divert or bleed offa sufficient portion of the secondary flow to atmosphere that the flowreaching the plasma jet does not adversely impact on its operation. Tothis end, the ports are preferably canted away from the plasma jet at asmall acute angle, as shown. On the other hand, on start up and duringpiercing, very high flow rate causes the secondary gas flow to blow bythe vents 72g with little diversion of the flow to atmosphere throughthem. On shut down, as the secondary gas pressure in the path 50 andprechamber 58 drops, the vents 72g provide a vent path to atmosphere toassist in rapidly decreasing the secondary gas pressure. Note thatbecause the face piece 72d is a separate component of the torch, if itbecomes worn or damaged it can be replaced without replacing the entirecap 72.

By way of illustration, but not of limitation, a torch 10 having arating of 15 amperes has an overall diameter of about 1.5 inches, exitorifice 72i, has a diameter of about 0.060 inch, a swirl ring 60 has aninside diameter of 0.300 inch and outside diameter of 0.400 inch and 6equiangularly spaced, off-center holes 64 with a diameter of 0.016 inch.The flow restriction orifice 56 has a diameter of 0.030 inch and theprechamber 58 has an internal volume of approximately 0.500 sq. inches.The exit orifice has a radial flow path from the swirl ring 60 to theouter diameter of the exit orifice 72i of about 0.008 inch. The vents72g are twelve in number and have a diameter of 0.16 inch.

Another principal feature of this invention is the use of a secondarygas that is a mixture of a non-oxidizing gas--such as nitrogen, argon,helium, or any of the inert gases--and an oxidizing gas such as oxygenor air, where the oxidizing gas comprises at least 40% of the mixture,measured by flow rates. In the preferred form, with oxygen as the plasmagas, the secondary gas is formed of a mixture of oxygen and nitrogen(argon) with their respective flow rates in a ratio in the range ofabout 2:3 to about 9:1, and preferably about 2:1. The 2:1 preferredratio is almost exactly opposite to the ratio of these gases formingair. The gases are commercially pure and are substantially free of waterand oil. When these gases are used in this ratio as a shield gas asdescribed above with respect to FIGS. 3A, 3B, 3C and 3D, the cuttingspeed of the torch in mild steel has been found to increasedramatically. In addition, the cut angle changes from 1° to 2° positivewith an air shield to about 0°, or generally perpendicular to theworkpiece. Further the top dross can be controlled to a point where itis negligible.

The exact flow ratio for the oxygen and nitrogen flows forming thesecondary gas can be determined empirically by cutting with the torchand adjusting the flows until the cut angle or other cut parameter orparameters are optimized. In making these adjustments it has been foundthat an increase in the oxygen flow will increase the cutting speed (upto about three times the speed of a conventional cutting speed with nogas shield). It also causes the cut angle to become very negative, up to4° to 5° for a pure oxygen flow. Also, the cutting surface becomesincreasingly rough and it exhibits a zig-zag pattern. The reason forthese effects is not well understood, but it is believed that a richoxygen environment surrounding the plasma jet assists a chemicalreaction between the metal and the oxygen which releases thermal energythat assists in melting the metal. The cut angle may also be explainedas an effect of the oxygen secondary flow on the shape of the plasma jet34.

Increasing the nitrogen flow, on the other hand, appears to influencecutting speed only to the extent such an increase is at the expense ofthe oxygen flow rate. A pure nitrogen flow is characterized by a cuttingangle that is 2° to 3° positive, a smooth cut surface, and some increasein dross as compared to cutting with no shield gas. It has been foundthat by changing the oxygen-nitrogen mixing ratio and the totalsecondary gas flow one can adjust the cutting angle from about positivethree degrees to negative three degrees. An increase in the oxygen inthe mix and an increase in the total flow makes the cut angle morenegative. Thus, the cutting angle can be tuned to a desired value simplyby changing the secondary gas mixture, rather than by changing thegeometry of the torch, as was the case in the past. Also, when the cutangle is maintained at a zero or negative value, top dross issubstantially eliminated.

The oxygen rich secondary gas mixture of the present invention alsoimproves the piercing capabilities of the torch 10. A pierced hole madewith an oxygen rich secondary gas according to the present invention iscleaner and can penetrate greater thicknesses of sheet metal thanidentical torches operating with different mixtures such as air.

FIG. 4 shows the gas flow control circuit 44 which controls the flow ofplasma and secondary gases from sources 42 and 46 to the inlets 10a and10b of the torch 10. The plasma gas, which for the purposes of thisdiscussion will be taken as oxygen, flows from the source 42 through anitrogen/oxygen solenoid selector valve SV15 (normally in the oxygenselect position). It is then split into a plasma gas flow along line 76and a secondary gas (oxygen portion) flow along line 78 to the oxygenfeed line 86 in the secondary gas section 44b of the control 44. Thesecondary gas supply 46 feeds a conduit 82 which has a branch line 84 tothe switch SV15 in the event that nitrogen is desired as the plasma gas.Pressure switches PS1 and PS2 in lines 76 and 82 do not allow the plasmacutting system to work if the pressure falls below a preset value.

In the preferred form shown, using oxygen as the plasma gas and amixture of oxygen and nitrogen as the secondary gas, three feed lines76, 78 and 82 are used. Each has a flow meter FM1, FM2, and FM3,respectively, and a pressure gauge PG1, PG2, PG3 connected in serieswith the flow meter. The flow meters ensure precise settings of the flowrates of both plasma and shield gas flows. Three bypass solenoid valvesSV8, SV9 and SV10 are connected in parallel with the three flow meters,respectively. These valves are three way valves that are normally opento the bypass line. This serves to protect the flow meters duringtransient times and during steady state three valves are closed allowingthe flow measurement.

Three normally closed solenoid valves are connected in parallel witheach other at the downstream side of the flow meter for each line 76, 78and 82. Each solenoid valve is followed by a needle valve. Each set ofthese solenoid valves has one that controls the preflow, one valve thatcontrols the operating flow, and a third valve that provides for a quickcharge. For oxygen plasma line 76, the preflow valve is SV2, theoperating valve is SV1 and the quick charge valve is SV3. The associatedneedle valves are MV2, MV1 and MV3, respectively. For the oxygensecondary gas line, these three solenoid valves are SV5, SV4 and SV16,followed by needle valves MV5, MV4, and MV8, respectively. For thenitrogen secondary gas line, these solenoid valves are SV7, SV6, andSV17, followed by associated needle valves MV7, MV6, and MV9respectively. The outputs from the valves SV4, SV5, SV6, SV7, SV16, andSV17 are combined to single secondary gas lead 86 connected to thesecondary gas inlet 10b at the torch. The output of the oxygen andnitrogen secondary gas lines is therefore combined into a single flow tothe torch.

The gas control circuit 44 also include four three way vent valves thatare each normally open to atmosphere. They are also electricallyactuated solenoid valves. Venting valve SV11 is connected to the oxygenplasma gas line at a gas console 88 that houses the gas control circuit44. A like venting valve SV13 is also connected in line 80, but at thetorch. This valve has a flow restricting orifice CO1 in the vent passageleading to atmosphere. It controls the decay of the plasma gas pressurein the nozzle on shut down. It is adjusted so that the gas pressuremaintains the arc while the current is on, but rapidly dissipates theplasma gas pressure when the current is cut off. In the secondary gasfeed line 86, a vent valve SV12 is connected to the line at the console88 and a like valve SV14 is connected in the line at the torch. The gascircuit 44 also has pressure gauges PG4 and PG5 connected at the console88 to the combined outputs from the preflow, operating flow and quickdischarge valves. PG4 reads the oxygen plasma pressure on line 80, PG5reads the secondary gas pressure on line 86.

During piercing the preflow valves are energized to open, with theoperating valves and quick charge valve closed during most of thepreflow. In this situation, the needle valves MV5 and MV7 control themix ratio of the oxygen and nitrogen flows forming the secondary gas. Asdiscussed above, this ratio is preferably set at about 2:1, butadjustments can be made to optimized the flow for the given operatingconditions and to optimize varying cut parameters. Also, the preflowthrough the valves SV5, MV5, SV7 and MV7 is set at a flow rate manytimes greater than the operating flow rate set by valves SV4, MV4, SV6,MV6. A typical value for the total secondary gas preflow is 120 scfh,and 20 scfh for the operating flow. Suitable three way solenoid valvesare manufactured by Automatic Switch Company under Model No. AFP33183 orby MAC Valves Inc. under Model No. 111B-111BAAA. The valves are allcontrolled by a central microprocessor 90 that is programmed to operatethe gas control circuit 44 in the manner illustrated by the timingdiagram of FIG. 5.

FIG. 5 it illustrates the operating state of all of the valves in thecircuit 44 during a full cycle of operation of the torch 10, from t₀when a start signal is given to the system by an operator to a completeshut off of the arc current and gas flows at the end of t₆. FIG. 5 alsoshows the corresponding arc current, voltage, and gas pressures at thenozzle (in the plasma arc chamber) and the secondary shield gas pressuremeasured at the prechamber 58 between caps 66 and 72.

As soon as the start command is issued, the three preflow solenoidvalves SV2, SV5 and SV7 are energized to open. The four venting valvesSV11, SV12, SV13 and SV14 are energized to closed position (they arenormally open). The three quick charge valves SV3, SV16 and SV17 arealso energized at the same time. The quick charge valves bring thenozzle and shield gas pressures up to their full preflow values in timet₁ for the plasma gas and in time t₂ for the shield gas. The quickcharge valves work to quickly charge lines 80 and 86 because they allowthe flows to bypass the flow restriction in the preflow and operatingflow branches. They allow a sudden, step function increase in the flow.The preflow continues for a total elapsed time of 1 to 2 seconds, longenough to stabilize the preflows. As shown in FIG. 5, high voltagespikes 91 are applied at a high frequency to the torch after about 1second of preflow to initiate a pilot arc, shown at 92. Once breakdownoccurs for the pilot arc, the voltage falls.

At the transfer of the arc to the workpiece, the current is ramped up asshown at 94, to its operating level 96 at the completion of thetransfer. The voltage drops on transfer and the gas pressure rises asthe plasma gas in the torch at the nozzle is heated to extremely hightemperatures and the gas flow is choked at the nozzle orifice 28a.During the transfer, piercing occurs. To provide the high velocity gasshield of the present invention, the large secondary gas preflow ismaintained for about 60 ms after the beginning of the transfer. Thishigh flow rate secondary gas preflow blows away molten metal splashedupwardly toward the torch before it can reach the torch itself. The flowsurrounds the plasma jet and is radially inwardly directed. It interactswith the jet, but most of the flow turns and flows radially away fromthe jet sweeping outwardly and downwardly in the region between theworkpiece and the lower end of the torch. It creates a moving, cool gasboundary between the cap 72 and the workpiece. This strong flow existsduring piercing, but is greatly reduced during normal cutting. Duringcutting the mechanical shielding of the cap 72 protects the nozzleagainst double arcing.

After about 50 ms from the beginning of the transfer the plasma gasquick charge valve SV3 is reopened for a time t₂ to bring the plasma gasflow up to its full operating valve quickly. Also after 50 ms from thebeginning of the transfer operating flow valves for both the plasma andshield gas open SV1, SV4, SV6. After a time t₃ from transfer, the twoshield line vent valves SV12 and SV14 are opened briefly, for time t₄ asshown, to assist the pressure in the secondary line in falling to alevel consistent with a much lower operating flow. This is the secondarygas quick discharge. The valves remain in these operating positionsduring operation except that the three flow meter bypass valves areenergized about 300 ms after the commencement of transfer. This is afterthe flows reach their steady state values. To stop operation of thetorch, a STOP command (i) deenergizes and closes the three operatingvalves SV1, SV4 and SV6, (ii) deenergizes the four vent valves to openthem to atmosphere and thereby facilitate a quick discharge of theplasma and secondary gases, and (iii) deenergizes the flow meter bypassvalves. From the STOP command to the end of t₆ the arc current is rampeddown. At the end of t₆ it is cut off completely. There is a smallresidual pressure at the nozzle, but it rapidly dissipates so that thereis substantially no strong swirling gas flow in the plasma chamber atcurrent off, end of t₆. This condition has been found to be highlyconducive to reducing electrode wear.

There has been described an apparatus and process for a plasma arc torchthat protects the torch against double arcing on piercing and duringcutting of sheet metal. There has also been described a gas shield forthis protection using a very high flow of a secondary gas during thepreflow only. The invention also describes an oxygen-rich secondary gasflow for the preflow on piercing and the operation flow that produces asignificantly faster and higher quality cut than heretofore attainableusing shielded torches or high definition torches. There has also beendescribed a system for precisely controlling the gas flows to the torch,both primary and secondary, both preflow and operating flow, so that ahigh flow rate secondary gas can provide a gas shield, but the operatingflow is low enough that it does not detract from the cut quality. Thiscontrol is sufficiently rapid that the plasma arc is maintained wellunder control despite large and sudden charges in the gas flows throughthe torch. There has also been described a nozzle which particularlyadapted to the high temperature, water-cooled operating environment of ahigh definition torch.

While this invention has been described with respect to its preferredembodiments, it will be understood that various modifications andvariations will occur to those skilled in the art from the foregoingdetailed description and the accompanying drawings. For example, whilethe invention has been described with respect to a high definition torchwith two different gases, one oxidizing and the other non-oxidizing, theinvention can be used in conventional torches and with a single type ofgas. Use of a single gas, however, provides only the gas shieldadvantages, not the increased cutting speed, cut quality, or tunabilityinherent in the oxygen-rich gas composition preferred for use as thesecondary gas for applications where an active gas such as oxygen isoptimal for use as the plasma gas. Also, while a network of valves andvents actuated electronically under the control of a microprocessor hasbeen described, other arrangements can be used to supply the plasma andsecondary gases in the right mix, at the proper times, and with a highdegree of precision in the timing. For example, rather than opening anextra line using a valve to quick charge, an independent source of highpressure gas can be suddenly and briefly opened to the main feed line toprovide a step function increase in the flow rate. Also, while a flowrestriction orifice and prechamber are used to create a pressure dropand enhance flow uniformity, other arrangements are possible. Further,while the invention has been described with respect to a swirl ring inthe secondary gas flow path, a non-swirling secondary gas flow can alsobe used, although with some loss of performance. These and othermodifications and variations are intended to fall within the scope ofthe appended claims.

What is claimed is:
 1. A method of operating a plasma arc cutting systemincluding a plasma arc torch having a body, an electrode and a nozzlemounted at a first end of the body in a mutually spaced relationshipthat defines a plasma chamber in which a plasma arc is formed, thenozzle having a central passage and a nozzle exit orifice through whichthe plasma arc passes, the method comprising:directing a plasma gas flowfrom a plasma gas inlet to the plasma chamber; forming a secondary gasflow as a mixture of a non-oxidizing gas and at least 40% of anoxidizing gas, as measured by flow rate; directing the secondary gasflow from a secondary gas inlet to a secondary gas flow path; alteringthe secondary gas flow in the secondary gas flow path to facilitate theinteraction of the secondary gas with the plasma arc; and directing thesecondary gas flow from a secondary gas flow path through secondary gasflow exit orifice and onto the plasma arc as the plasma arc passesthrough file nozzle exit orifice.
 2. The method of claim 1 wherein saidnon-oxidizing gas is selected from the group consisting of nitrogen andargon and said oxidizing gas is selected from the group consisting ofoxygen and air.
 3. The method according to claim 2 wherein said ratio isabout 2:1, oxidizing gas flow to non-oxidizing gas flow.
 4. The methodaccording to claim 1 wherein the torch is a high definition torch. 5.The method of claim 1 wherein said ratio is adjusted to produce a kerfcut angle that is generally perpendicular to the workpiece withnegligible top dross.
 6. The method of claim 1 wherein the altering stepcomprises introducing a swirling movement to the secondary gas flow. 7.The method of claim 6 wherein the altering step further comprisesdirecting the secondary gas flow through a swirl ring for introducingthe swirling movement to the secondary gas flow.
 8. The method of claim1 further comprising mounting a water-cooled cap on said bodysubstantially enclosing the outer surface of the nozzle.
 9. The methodof claim 8 further comprising mounting a secondary gas cap on said bodyin a spaced relationship with said water-cooled cap to define a portionof said secondary gas flow path which includes the secondary gas flowexit orifice.
 10. The plasma arc cutting system of claim 1 wherein thecontrol network includes conduits, valves, meters and vents.
 11. Amethod of operating a high definition plasma arc cutting systemincluding a plasma arc torch having a body, and an electrode and anozzle mounted in an end of the body in a mutually spaced relationshipto define a plasma chamber in which a plasma arc is formed, the nozzlehaving a central passage and a nozzle exit orifice through which theplasma arc passes, the method comprising:directing a plasma gas flowfrom a plasma gas inlet to the plasma chamber; forming a secondary gasflow as a mixture of a non-oxidizing gas, selected from the groupconsisting of nitrogen and argon, and at least 40% of an oxidizing gas,selected from the group consisting of oxygen and air, as measured byflow rate; directing the secondary gas flow from a secondary gas inletthrough a swirl ring for introducing a swirling movement to thesecondary gas flow in order to facilitate the interaction of thesecondary gas with the plasma arc; and directing the secondary gas flowfrom the swirl ring through a secondary gas flow exit orifice and ontothe plasma arc as the plasma arc passes through the nozzle exit orifice.12. The method according to claim 11 wherein said ratio is about 2:1,oxidizing gas flow to non-oxidizing gas flow.
 13. The method of claim 10wherein the secondary gas flow is not substantially ionized and ishighly uniform.
 14. The method of claim 10 further comprising mounting awater-cooled cap on the body substantially enclosing the outer surfaceof the nozzle.
 15. The method of claim 14 further comprising mounting asecondary gas cap on said body in a spaced relationship with thewater-cooled cap to define a portion of the secondary gas flow pathwhich includes the secondary gas flow exit orifice.